1. Field of the Invention
This invention relates to a system for determining whether an IIR (infinite impulse response) digital filter, in which the sum of the magnitudes of its filter coefficients is more than unity, is unstable and, if so, flattening the phase response of the filter to render it stable. This system is particularly applicable to stabilizing the IIR digital filter of deghosting apparatus employed in a digitally-implemented television receiver for reducing multipath signal components of a received transmitted television signal.
2. Description of the Prior Art
Known in the art is so-called deghosting apparatus, which is situated in a television receiver at a given location for eliminating or, at least, substantially reducing multipath distortion from a video signal derived from a broadcast television channel received thereby. One example of such deghosting apparatus, which is implemented in digital form, is disclosed in U.S. Pat. No. 5,065,242, which issued to Dieterich et al. on Nov. 11, 1991, the teachings of which are incorporated herein by reference. This type of deghosting apparatus has been proposed for use by either a standard (NTSC) or a high-definition (HDTV) digital television receiver responsive to the appropriate one of simulcast NTSC or HDTV television signals received thereby.
Specifically, the deghosting apparatus disclosed in U.S. Pat. No. 5,065,242 incorporates a training signal which is a finite duration sequence of repetitions of a 2.sup.n -1 pseudorandom sequence. The 2.sup.n -1 pseudorandom sequences are mapped into 2.sup.r sample intervals at either the transmitter or the receiver to facilitate fast Fourier transform processing. Power spectra of the received and mapped training signal and a stored version of the training signal are deconvolved with the deconvolution coefficients produced in logarithmic form. The logarithms are inverse Fourier transformed to produce delay coefficients related to the time of occurrence of the multipath signals, which coefficients are utilized to program variable delay circuitry in an adaptive filter to cancel the multipath components. Such programming is required because the multipath components depend on the particular location of each individual digital television receiver.
This adaptive filter includes an IIR digital transversal filter, which may comprise a tapped delay line implemented by a relatively large shift register (e. g., a 256 or 512 stage shift register), with only a relatively small number of these stages (e. g. 25 to 127 stages) having non-zero real or complex multiplier coefficients associated with the taps thereof. However, the particular ones of the stages of the shift register having non-zero multiplier coefficients, as well as the quantized value of each of them, are programmable in order to provide the maximum cancellation of the multipath components received by that particular digital television receiver, without causing the IIR filter itself to become unstable (due to excessive positive feedback).
It is known that an IIR filter will always be stable so long as the sum of the magnitudes of each of its multiplier coefficients is less than unity. Should the sum of the magnitudes of each of the IIR filter's multiplier coefficients be greater than unity, it may or may not be stable in any given case. Thus, in order to insure IIR filter stability in all cases, it has been the practice in the past to scale the value of each of the multiplier coefficient magnitudes by the same fractional value (thereby not changing their relative values) whenever the sum of their unscaled values exceeds unity, so that the sum of the magnitudes is below unity. However, such scaling reduces the degree to which the deghosting apparatus is capable of cancelling the multipath distortion.
The teachings of the present invention makes it possible in nearly all cases to employ a sum of the magnitudes of the IIR filter multiplier coefficients greater than unity without the possibility of the IIR digital filter becoming unstable. When the teachings of the present invention are applied to deghosting apparatus, the degree of cancellation of multipath distortion is maximized. However, the benefits of the teachings of the present invention are applicable to the design of any IIR digital filter, regardless of its particular use.